使用二进制搜索进行模板递归

Question

Consider this working code I wrote:

#include <iostream>

constexpr int MIN_FOO = 0,  MAX_FOO = 100;

template <int N>
void foo() {std::cout << "foo<" << N << ">() called.\n";}

template <int N>
void foo (char, double, bool) {std::cout << "foo<" << N << ">(char, double, bool) called.\n";}


template <int Low, int High, typename... Args>
void searchFooBinary (int key, Args... args) {
//  if (LOW > HIGH) {std::cout << "Error with searchFooBinary.\n";  return;}
    constexpr int Mid = (Low + High) /2;
    if (key == Mid)
        foo<Mid>(std::forward<Args>(args)...);  // Want to generalize this using 'f'.
    else if (key < Mid)
        searchFooBinary<Low, Mid - 1>(key, std::forward<Args>(args)...);
    else
        searchFooBinary<Mid + 1, High>(key, std::forward<Args>(args)...);
}

template <typename... Args>
void executeFooBinarySearch (int n, Args... args) {
    searchFooBinary<MIN_FOO, MAX_FOO>(n, std::forward<Args>(args)...);
}

int main() {
    executeFooBinarySearch(99);
    executeFooBinarySearch (99, 'a', 1.5, true);
}

So foo is a template function and here a run-time int is passed and searchFooBinary uses a binary search to find the correct int value for the template argument. So far so good, but I don't want to write this binary search function for every new function like foo . How do I generalize the use of foo in searchFooBinary to the more general f ? If a template function pointer is not allowed, what would be a workaround to achieve this goal?

Answer 1

How about using a functor:

#include <iostream>

constexpr int MIN_FOO = 0,  MAX_FOO = 100;

struct Foo
{
    template <int N>
    void operator() (char, double, bool) {std::cout << "Foo<" << N << ">(char, double, bool) called.\n";}
};

struct Bar
{
    template <int N>
    void operator() () {std::cout << "Bar<" << N << ">() called.\n";}   
};


template <int Low, int High, typename Fun, typename... Args>
void searchBinary (int key, Fun f, Args... args)
{
    constexpr int Mid = (Low + High) /2;
    if (key == Mid)
    {
        f.template operator()<Mid>(std::forward<Args>(args)...);
    }
    else if (key < Mid)
    {
        searchBinary<Low, Mid - 1>(key, f, std::forward<Args>(args)...);
    }
    else
    {
        searchBinary<Mid + 1, High>(key, f, std::forward<Args>(args)...);
    }
}

template <typename Fun, typename... Args>
void executeBinarySearch (int n, Fun f, Args... args)
{
    searchBinary<MIN_FOO, MAX_FOO, Fun>(n, f, std::forward<Args>(args)...);
}

int main()
{
    executeBinarySearch (99, Foo(), 'a', 1.5, true);
    executeBinarySearch (99, Bar());    
}

output

Foo<99>(char, double, bool) called.
Bar<99>() called.

Live example: http://ideone.com/fSoG5B